Regarding Jim's thought on venting the engine compartment with a "boxed radiator" ...
Oddly enough I had two new trucks in my shop with the front grilles removed, both had active aero grille shudders. I didnt have any time to test them electronically, but watching their movement my guess is that they are controlled with stepper motors. Using a engine compartment temp sensor and a vehicle speed sensor, a simple code could be written to control the shudder position.
For a full belly pan'd car Im picturing this unit being mounted directly under the engine.
At high speeds, but low load (ie: highway cruising) the vents could be totally closed for max efficiency.
As load and heat increase the vents could open at the rear to vent the underhood air. And low speeds (traffic, or low speed autocross maybe?) where the engine compartment temp is high from lack of airflow, the vents could be opened to create a ram-air effect.
Surely this could be implemented anywhere on the car, but this is where my head is at currently :shocking: